Inertial sensors are usually encapsulated using silicon cap wafers by sealing-glass bonding. Such a manufacturing method is described in German Patent Application No. DE 19700734. The electrical contacting is accomplished by cut-outs, produced specifically for this purpose, in the cap wafer. When using sealing-glass bonding, a long-lasting hermetic and reliable bond is only made possible with relatively wide bonding frames.
The use of anodically bondable cap wafers permits a significant reduction in the width of the bonding frames, and with that, the surface-area requirement per sensor chip. Therefore, anodically bondable cap wafers are used in newer encapsulation techniques like, for example, the MPT (micro packaging technology) put forward in German Patent Application No. DE 10104868. Here, anodically bondable composite wafers made up of a silicon carrier substrate and a sufficiently thin glass substrate are used as cap wafers. The glass substrate must be selected to be thin enough that the ability of the cap wafer to be sawed by the series equipment usually used is ensured.
Because the composite cap is difficult to structure, when working with MPT, for example, the electrical accesses are produced from the back side of the sensor substrate with some degree of technological complexity. This includes, first of all, the internal wiring of the sensor structures, which differs sharply from the previous series processes. Moreover, the complicated backside contacting requires planarization of the composite cap, edge grinding and back grinding of the sensor substrate, stress relief of the sensor substrate, Si-DRIE (deep reactive ion etching) of isolation trenches, and finally a costly refill technique, including the producing of bonding pads on the back side of the sensor substrate.